What is wrong with humans...? Honestly? How did humans survive with all of these more advanced races around? You look one way and the dwarves have adapted to living in the mountains and building things for durability. Another way the elves have studied the arcane and near perfected it. The gods took pity on the halflings and they kept to themselves. The gnomes protected themselves with arcane engineering and other witty ideas. But you? Human? No gods protected you. You have almost no evolutionary specialization. Why are humans still around? Well some people call it "Human Determination" and those people are looking at it optimistically. Others like to call it "The only reason humans have lasted this long."

Once a long rest you may take a critical success on any one roll once a long rest. Your DM then may determine a repercussion that may take place. This can be damage to you and your surroundings, an attack of opportunity, exhaustion, a curse, a disease, and many other things. These can be in absolutely chaotic and disastrous ways or a simple opening that an opponent spots. 

Example: "A human artificier is tinkering about with their new project when suddenly while inside it they cross a wire wrong. The artificier electrocutes themself but as they are launched back by the shock the project whirrs to life. They succeeded but they took damaged in the process.